The use of a so-called "chassis dynamometer" with inertial weights to measure the acceleration and deceleration performance of an automobile or other vehicle has long been known. An article by Knudsen R. F., "Inertia Electronically", ISA Journal, April 1958, Vol. 5, No. 4, pp.52-54 describes the use of electrical torque to simulate drum inertia and thereby avoid the need to add or remove large bulky weights from the dynamometer shaft whenever it was desired to alter the inertia of the rolls or drums of the dynamometer. Knudsen derived the inertia by differentiating the speed signal from a drum tachometer, amplifying it and feeding it back as an additional torque control signal to a dynamometer torque control circuit. Gunnel and Shaw have described in patent specification No. GB-A-1297813 a drum dynamometer testing machine for the tyres and brakes of land vehicles and aircraft in which the test tyre and wheel was braked and a brake effort signal was processed and fed as a control signal to determine the torque of a Ward-Leonard dynamoelectric machine that drove the drum. Friction and windage losses were compensated for by feeding a derived speed signal through a function generator as another control signal to be combined with the brake-effort related control signal and fed to the dynamo-electric machine. The result was again to give an apparent or effective inertia to the drum that was its true mechanical inertia multiplied by an adjustment factor. A practical machine of this kind was described by J. B. Roberts entitled "An Advanced High-Speed Dynamometer for Testing Aircraft Tyres, Wheels and Brakes" and read before the Applied Mechanics Group of the Institution of Mechanical engineers on the Apr. 10, 1974. A modified machine described by Thomas in Patent specification No. GB-A-1604320 monitored the change in kinetic energy of the drum and associated mechanical parts as the drum decelerated and derived a control signal for the drum drive motor to vary the apparent or effective inertia of the drum. But these machines of the "inertia simulation" kind merely used an electrical control system to simulate the removable weights of a conventional chassis dynamometer of the 1950's and embodied the assumption that it was satisfactory to monitor energy dissipation purely passively in terms of the inertia of the rotating parts and the change in rotational velocity. It has, however, been found that inertia simulation gives only limited accuracy and repeatability, particularly where the quantity of energy to be absorbed or dissipated is only a small proportion of the initial rotational energy of the moving parts, and it is an object of the present invention to provide a dynamometer that can be made of improved accuracy.